Alkanoic acid recycle system in process for catalytic production of an alkenyl alkanoate from an alkene and an alkanoic acid

ABSTRACT

1. IN A PROCESS WHICH COMPRISES INTRODUCING A LIQUID LOWER ALKANOIC ACID SELECTED FROM THE GROUP CONSISTING OF ACETIC ACID AND PROPIONIC ACID AND A CONTINUOUS STREAM OF FIXED GAS COMPRISING PREDOMINANTLY A LOER ALKENE OF UP TO ABOUT SIX CARBON ATOMS INTO A HEATED VAPORIZATION APPARATUS; CONTINUOUSLY VAPORIZING AT LEAST A PORTION OF SAID ALKANOIC ACID INTO SAID ALKENE STREAM IN SAID APPARATUS; WITHDRAWING FROM SAID APPARATUS A GASEOUS MIXTURE COMPRISING PREDOMINANTLY SAID ALKENE AND ALKANOIC ACID; PASSING SAID GASEOUS MIXTURE IN ADMIXTURE WITH MOLECULAR OXYGEN THROUGH A CATALYTIC CONVERTER CONTAINING A GROUP VIII NOBLE METAL CATALYST; REACTING SAID ALKENE, ALKANOIC ACID, AND OXYGEN IN THE VAPOR PHASE, IN SAID CONVERTER IN THE PRESENCE OF SAID CATALYST TO FORM AN ALKENYL ALKANOATE; CONTINUOUSLY WITHDRAWING FROM SAID CONVERTER A GASEOUS PRODUCT COMPRISING SAID ALKENE, AN ALKENYL ALKANOATE, ALKANOIC ACID, AND HIGH-BOILING BYPRODUCTS; SEPARATING SAID GASEOUS PRODUCT INTO A FIXED-GAS COMPONENT COMPRISING PREDOMINANTLY SAID ALKENE AND A LIQUID COMPONENT COMPRISING ALKANOIC ACID, AN ALKENYL ALKANOATE, AND HIGH-BOILING BY-PRODUCTS; RECOVERING SAID ALKENYL ALKANOATE AS DISTILLATE FROM SAID LIQUID COMPONENT BY DISTILLATION WHILE RECOVERING ALKANOIC ACID AND SAID HIGH-BOILING BY-PRODUCTS AS THE RESIDUE STREAM FROM SAID DISTILLATION; AND UTILIZING SAID RESIDUE STREAM AS A SOURCE OF ALKANOIC ACID TO BE RECYCLED TO SAID CATALYSTIC CONVERTER, THE IMPROVEMENT WHICH COMPRISES: STRIPPING ALKANOIC ACID FROM SAID DISTILLATION RESIDUE STREAM, AND INCORPORATING SAID STRIPPED-OUT ALKANOIC ACID INTO SAID GASEOUS MIXTURE WHICH IS TO BE PASSED THROUGH SAID CONVERTER, BY COUNTERCURRENTLY STRIPPING SAID DISTILLATION RESIDUE STREAM WITH THE GASEOUS MIXTURE COMPRISING ALKANOIC ACID AND SAID ALKENE BEING WITHDRAWN FROM SAID VAPORIZATION APPARATUS.

United States Patent 3,840,590 ALKANOIC ACID RECYCLE SYSTEM IN PROCESSFOR CATALYTIC PRODUCTION OF AN ALKE- NYL ALKANOATE FROM AN ALKENE AND ANALKANOIC ACID Gene J. Fisher and Edward N. Wheeler, Corpus Christi,Tex., assignors to Celanese Corporation, New York,

No Drawing. Continuation-impart of abandoned application Ser. No.109,046, Jan. 22, 1971. This application Sept. 12, 1972, Ser. No.288,467

Int. Cl. C07c 67/04, 67/06 U.S. Cl. 260-497 A 8 Claims ABSTRACT OF THEDISCLOSURE In a process for producing an alkenyl alkanoate such as vinylacetate by passing an alkene such as ethylene together with an alkanoicacid and molecular oxygen over a nobel metal catalyst, followed byrecovering a crude alkenyl alkanoate fraction and a fraction comprisingthe alkanoic acid and high-boiling reaction by-products from the reactoreffiuent, the alkanoic acid is recovered from the alkanoicacid-containing fraction just named by countercurrently stripping itwith a heated mixture comprising the alkene and alkanoic acid preparedby vaporizing the alkanoic acid in the presence of a gas comprising someor all of the alkene which is to be introduced into the catalyticreactor. The countercurrent stripping is preferably accomplished in astripping tower which is surmounted by a rectification section into thetop of which is introduced fresh alkanoic acid free of the high-boilingbyproducts of the alkenyl alkanoate synthesis reaction.

Related Patent Applications This is a continuation-in-part of patentapplication Ser. No. 109,046, filed Jan. 22, 1971, and now abandoned.

Background of the Invention This invention relates to the production ofunsaturated esters by the catalytic reaction of an olefin and acarboxylic acid by a process which comprises passing the olefin and theacid, in admixture with molecular oxygen, over a catalyst comprising anoble metal, e.g. palladium. In particular it relates to the productionof vinyl acetate from ethylene and acetic acid in admixture withmolecular oxygen in the presence of a solid catalyst comprising a noblemetal. More specifically it relates to a method for economicallyseparating high-boiling reaction by-products from unconverted aceticacid or other alkanoic acid which is contained in the reactor effluentand which it is desired to recycle to the reactor substantially freefrom such high-boiling by-products.

Recently-developed processes for reacting ethylene, acetic acid, andmolecular oxygen in the presence of a solid catalyst comprising a noblemetal include those described in British Pat. No. 1,163,850 and U.S.Pats. Nos. 3,190,912 and 3,275,680. The catalyst typically comprises anobel metal, e.g. a Group VIII nobel metal or a salt or oxide thereof,including especially palladium or rhodium. British Pat. 1,163,850 dealswith an improved catalyst comprising metallic palladium and metallicgold, and this patent and U.S. Pat. 3,275,680 describe a'mode ofoperation in which, in addition to the noble metal, an alkali metalacetate is present in the catalyst system. Advantageously the catalystis disposed on a support such as an aluminum oxide-bentonitecomposition, alumina, or carbon.

A particularly favorable mode of operation in the above-describedprocesses comprises passing a vaporphase mixture comprising ethylene,acetic acid, and

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molecular oxygen at elevated temperature and pressure through acatalytic converter containing a bed of the solid catalyst, typically inthe form of beads comprising a support material impregnated with one ormore noble metals, with an alkali metal acetate also being present. Thegaseous reaction product discharged from the converter contains vinylacetate, unconverted acetic acid and ethylene, lesser quantities ofunconverted oxygen, and inert fixed gases such as carbon dioxide andnitrogen. Acetaldehyde may also be present.

The reactor effluent gas just described is separated, by methods whichare conventional but which are not pertinent to the present invention,into a recycle gas stream comprising the unreacted ethylene and at leasta portion of the other fixed gases and a crude liquid product outcomprising unreacted acetic acid as well as the vinyl acetate and otherliquefiable reaction by-products. The vinyl acetate and such volatileby-products as acetaldehyde and methyl and ethyl acetates are separatedfrom the acetic acid by, for example, a water-azeotropic distillation,as described in U.S. Pat. 3,551,299, in which the vinyl acetate isrecovered as an azeotropic distillate while the acetic acid is recoveredas distillation residue. U.S. Pat. 3,458,406 deals with additionaldistillation processing by which the vinyl acetate is further purified.

The residue from the azeotropic distillation just described comprisespredominantly acetic acid, which it is desired to recycle to the vinylacetate converter for further reaction. Typically the acetic acid beingincorporated into the vinyl acetate converter inlet streams is vaporizedby being contacted, at a suitably controlled temperature, with part orall of the ethylene-containing stream of fixed gases which is also beingforwarded to the converter as described in U.S. Pat. 3,190,912. Finalcomposition of the resulting vapor-phase mixture is, of course, adjustedas desired by incorporating molecular oxygen thereinto, either before orafter vaporization of the acetic acid. Alternatively, the prior art hasconsidered simply vaporizing the acetic acid in a flash evaporator, withthe resulting vapor being then mixed with ethylene and molecular oxygen.

Although distillation of the product mixture withdrawn from the vinylacetate converter is referred to in U.S. Pat. 3,275,680, for the purposeof removing reaction products therefrom before recycling the acetic acidcomponents to the vaporization step just discussed, there are twoproblems inherent in carrying out such a distillation by conventionalmethods, for which a simple and effective solution has heretofore beenlacking. First, the reaction by-products include high-boiling compoundsthe separation of which from acetic acid without either using stilltemperatures so high as to cause product contamination or else usingvacuum distillation with its attendant high cost is difficult at best.Second, it has now been discovered that, in addition to high-boilerswhich are of 10W volatility but not otherwise troublesome, there arealso present in the converter reaction product materials which, as wellas being of low volatility, are also polymerizable. One of these isacetoxyvinyl acetate. Other polymerizable vinyl compounds are alsopresent. These polymerizable high boilers present the additional problemof fouling of the surfaces of either a still reboiler or, alternatively,of the heat transfer surfaces and vapor discharge conduits of a simpleflash evaporator if an efiort is made to simply vaporize the acetic acidcontaining such materials without first removing them as bydistillation.

There is, accordingly a need for a simple and effective method forremoving the high-boiling reaction by-products from the recycle aceticacid before it is reintroduced into the vinyl acetate converter.

It is an object of the present invention to provide a method forremoving high-boiling reaction by-products from the recycle alkanoicacid, e.g. acetic acid, stream recovered from the reaction product in aprocess in which vinyl acetate or other alkenyl akanoate is formed bypassing ethylene or other alkene, acetic acid or other alkanoic acid,and molecular oxygen over a solid catalyst comprising a noble metal,especially a Group VIII metal, or its oxides or salts. It is anotherobject to provide a method for accomplishing this separation withimproved heat economy and with minimal polymer fouling of the interalsurfaces of the process apparatus employed. It is yet another object toprovide a method whereby said separation can be accomplished by the useof comparatively simple process apparatus. Other objects will beapparent from the following detailed description and example.

Summary of the invention In accordance with the present invention, asapplied for example to the production of vinyl acetate, the acetic acidcontent of the crude acetic acid cut resulting when vinyl acetate isseparated from the higher-boiling components of the catalytic vinylacetate converter efiluent is stripped out of said crude cut, andseparated from the higher-boiling reaction by-products which are presenttherein, by countercurrently contacting the crude acetic acid cut with aheated gas stream comprising ethylene which is to be fed into thecatalytic vinyl acetate converter. This is advantageously accomplishedby the steps of (a) injecting the ethylene into a heated acetic acidvaporizer, preferably into the heating element of the vaporizer wherebythe ethylene is saturated with the acetic acid and (b) introducing theresulting stream of vapor comprising ethylene and acetic acid into thebase of a countercurrent stripping column, while (c) introducing thecrude acetic acid out which is to be stripped into the top of thestripping column. From the top of the stripping column there is drawn avapor comprising ethylene and acetic acid, which is then forwarded tothe catalytic converter for production of vinyl acetate. The strippedliquid from the base of the stripping column, comprising the highboilingby-products of the vinyl acetate synthesis reaction, is disposed of asdesired. Advantageously it is allowed to pass into the vaporizer, fromwhich a residue comprising the high-boiling impurities is then withdrawnfor workup as desired, for example to recover additional quantities ofacetic acid.

It will be recognized that the ethylene introduced into the vaporizercan be pure, or it can be diluted with other gases, for example nitrogenand other inert gases which may be present in the ethylene-containinggas stream which is recycled to the catalytic converter from the primaryproduct recovery system in which the converter product is separated intoliquefiable components (e.g. vinyl acetate and acetic acid) and gaseouscomponents (e.g. ethylene and other fixed gases).

In a particularly suitable embodiment of the invention the strippingcolumn just described is surmounted by a rectification or absorptioncolumn section, into the top of which there is introduced a stream ofcomparatively pure acetic acid, i.e. acetic acid which does not containappre ciable quantities of the high-boiling reaction products which arepresent in the crude cut introduced into the top of the strippingsection. This reduces even further any contamination, by thehigh-boiling reaction by-products, of the acetic acid-ethylene vaporstream being fed into the vinyl acetate converter.

It will be recognized that this invention is not restricted in itsapplication to processes in which ethylene and acetic acid are beingreacted catalytically to form vinyl acetate. It can also be employed inchemically analogous reaction systems in which other lower alkenes suchas propylene or the butenes or other unsaturated compounds having morethan two and up to about six carbon atoms are reacted with acetic acidor other alkanoic acids, such as propionic acid specifically, in thepresence of molecular oxygen and'a noble metal-containing solid catalystto form unsaturated esters of the acid. In all such reactions theformation of high-boiling reaction by-products occurs as in the vinylacetate process, and the problem and its solution are the same as in thepresent invention. A specific example is the reaction of propylene andacetic acid to form allyl or isopropenyl acetate, with propylene glycolacetates being one of the higher-boiling by-products to be separatedfrom the recycle acetic acid by the present method. Other diacetoxypropanes will also be present. Dipropionoxy alkanes are formed inanalogous systems when propionic acid is employed in place of aceticacid.

Description of the preferred embodiments With reference to appaartus tobe employed, thecountercurrent stripping is preferably carried out in aplate-type column, simple perforated plates being particularly suitable,the individual plates being designed in accordance with conventionalmethods for the liquid and vapor loading to be employed. A packed columncan be used if desired, or a series of simple sparged vessels designedfor countercurrent contacting of a liquid stream with a vapor stream.Although fewer plates can be employed, if desired, at least about 5actual operating stripping trays are recommended. In' addition,particularly good results obtain when at least about 5 rectificationplates are superimposed on the stripping plates, with fresh acetic acidbeing introduced at the top of this rectification section for thepurpose of refluxing downward the compounds higher boiling than aceticacid which are present in the vapors evolved from the top of thestripping section. Although it is preferred, in this arrangement, tomaintain at least about 5 stripping trays between the vaporizer and thepoint at which the crude acetic acid to be stripped is introduced, thecrude acid can be introduced at a lower level if desired.

Although failure to do so does not defeat the basic purposes of theinvention, it is particularly recommended that the rectification sectionof the column be designed for minimum liquid entrainment in order toreduce the extent to which compounds less volatile than acetic acid arecarried out of the column, for example as mist, in the vaporousethylene-acetic acid mixture issuing from it. For example, it isadvantageous to use in the rectification section sieve-type traysdesigned and positioned for low liquid carryover. Such trays are easilydesigned for low pressure drop, which is also highly desirable in thissystem.

The acetic acid vaporizer can be, if desired, a separate piece ofapparatus from the stripping, or rectification and stripping, columnjust described. Advantageously it comprises a vessel equipped with aheater or reboiler by means of which liquid contained in the vessel isvaporized. Preferably the ethylene-containing gas stream with which the'vaporized acetic acid is to be admixed is introduced directly into thelower portion of this heater or reboiler, by which means the circulationrate is enhanced and the gas is readily saturated with the acetic acidvapors. This expedient also, by means of the steam-distillation effect,results in keeping to a minimum the temperatures prevailing in theheater or reboiler.

A particularly recommended arrangement of the column and the vaporizeris one in which the vaporizer and the column constitute a conventionalplate-column still, the vaporizer simply comprising the base of thecolumn with its attached reboiler (which is preferably of the thermalsiphon type) and with the ethylene-containing gas stream being injectedinto the base of the reboiler (on its so-called process side). With thisarrangement the stripped liquid leaving the bottom of the strippingcolumn drains into the liquid circulating through the reboiler, fromwhich it is tapped off, either continuously or intermittently, forworkup in, for example, a hight-emperature evaporator of the wiped-filmtype for separating residual acetic acid from the high-boiling compoundsand polymers which are present.

ly, the mixture of ethylene, inert gases, and oxygen which recycles fromthe vinyl acetate product recovery system back to the converter can beemployed, with the fresh ethylene makeup required by the tvinyl acetateprocess being incorporated into this stream either before or after thestripping operation. This stream comprises predominantly ethylene, withthe remainder being largely inert gases and a small amount (up to 2 or3% by volume) of oxygen. Exact composition of this stream is notpertinent to the present invention except that as a practical matter itis desirable, of course, to avoid high concentrations of oxygen with theresultant explosion hazard.

Temperature and pressure to be employed in the vaporizer and the columnmounted above it can vary over a wide range, although the pressure isconveniently maintained substantially the same as the pressure obtainingin the vinyl acetate converter. That is, the pressure is advantageouslyequal to the vinyl acetate converter pressure plus sufficient pressuredifferential to move the vapor evolved from the top of the columnthrough the piping andcontrol valves and similar apparatus intermediatebetween the column and the vinyl acetate converter. Typically thispressure is in the range of approximately 6 to 10 atmospheres absolute.With the pressure being set in this manner, heat is then supplied to thevaporizer in an amount which is preferably sufficient to vaporize theliquid contained in the vaporizer and evaporate into the gas streampassing through the column all of the acetic acid contained in therecycle acid stream introduced at the top of the stripping section andat least a portion of the acetic acid contained in the fresh acetic acidstream which, in the preferred embodiment of the invention, isintroduced into the top of the rectification section. The composition ofthe liquid contained in the vaporizer, i.e. the liquid circulatingthrough the reboiler, is preferably controlled so that its acetic acidcontent is at least roughly 50% by weight and preferably between about70% and about 80% by weight; this control is not critical, but isrecommended to prevent buildup of high-boiling compounds in the base tosuch a degree that the operating temperature becomes high enough tocause undesired thermal decomposition of some of these compounds andfouling of the vaporizer heat transfer surfaces.

Convenient operation of the vaporization apparatus under theabove-described conditions of temperature and pressure also obtainswhen, per unit weight of ethylenecontaining gas introduced into thevaporizer, there is introduced into the combined rectification andstripping sections of the column about 0.5 to 0.6 unit weight of aceticacid. It will be recognized that this is not a critical process controlparameter; that is, if the gas rate is insufficient to strip the desiredquantity of acetic acid out of the liquid phase, the gas rate or thevaporization temperature can be increased, and vice versa.

With regard to the relative proportions of the recycle acetic acidstream introduced at the top of the stripping section of the column andthe fresh acetic acid introduced at the top of the rectificationsection, satisfactory results obtain when the stripping andrectification sections of the column constitute together the equivalentof at least about 10 actual trays, 5 of which are in the strippingsection and 5 of which are in the rectification section, with about to50% of the total introduced acetic acid being introduced into the top ofthe rectification section and the remainder being introduced at the topof the stripping section (i.e. into the center of the total column).

The following example is given to illustrate the invention further. Itwill be recognized that many variations can be made therefrom within thescope of the invention.

6 EXAMPLE 1 The stripping of a crude recycle acetic acid stream,including rectification of the stripped vapors with fresh acetic acid,was carried out in a IO-tray bubble cap column which was 6 inches indiameter and which was equipped with a vertical thermosiphon reboilerhaving two tubes 6- feet long and 1 inch in diameter. The column wasoperated at a pressure of 6.7 atmospheres absolute. Temperature in thebase of the column was 265 F.; in the head of the column the temperaturewas 250 F. Onto the top tray of the column, fresh acetic free ofcontaminants other than about 0.1 weight percent water was introduced ata rate of 8.60 pounds per hour. Onto the fifth tray of the column therewas continuously introduced 35.2 pounds per hour of a crude acetic acidcut which had been obtained by azeotropically distilling vinyl acetateand lighter reaction by-products from the converter efliuent formed bypassing vapors of acetic acid along with ethylene and oxygen over asolid catalyst comprising metallic palladium on an amorphous silicasupport along with potassium acetate. This recycle acetic acid cutcontained, by weight, 95.4% acetic acid, 1.60% water, 0.44%non-volatiles including vinyl-type polymers, and 2.0 to 2.5%high-boiling by-products of the vinyl acetate synthesis reactioncomprising high-boiling acetate esters including acetoxyvinyl acetate.

Into the lower head of the column reboiler there was continuouslyintroduced a gas stream, composed of the ethylene-containing recycle gasstream resulting from the vinyl acetate synthesis operation togetherwith makeup fresh ethylene and comprising predominantly ethylene withlesser amounts of inert gases such as nitrogen along with small amounts(less than about 3 volume percent) of oxygen.

Heat was supplied to the reboiler at a rate such that there was evolvedfrom the head of the column, at the rate of 123.1 pounds per hour, agaseous mixture consisting of the above-described gas stream nowsaturated with acetic acid at 250 F., while there was produced from thebottom of the column a stripped residue stream amounting to 4.49 poundsper hour. The acetic acidethylene stream evolved from the head of thecolumn, which was suitable for incorporation into the feed system of thevinyl acetate converter, contained, by weight, less than 378 parts permillion of heavy ends, defined as compounds having a boiling pointhigher than that of acetic acid.

The residue stream withdrawn from the base of the column contained, byweight, 0.9% ethylene, 0.08% water, 77.5% acetic acid, 2.29% polymers,and 19.2% heavy ends. This withdrawn residue stream was subsequentlyprocessed through a high-temperature wiped-film evaporator to recover afinal residue comprising heavy ends still containing some acetic acid,and a distillate comprising a crude acetic acid.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. In a process which comprises introducing a liquid lower alkanoic acidselected from the group consisting of acetic acid and propionic acid anda continuous stream of fixed gas comprising predominantly a lower alkeneof up to about six carbon atoms into a heated vaporization apparatus;continuously vaporizing at least a portion of said alkanoic acid intosaid alkene stream in said apparatus; withdrawing from said apparatus agaseous mixture comprising predominantly said alkene and alkanoic acid;passing said gaseous mixture in admixture with molecular oxygen througha catalytic converter containing a Group VIII noble metal catalyst;reacting said alkene, alkanoic acid, and oxygen in the vapor phase, insaid converter in the presence of said catalyst to form an alkenylalkanoate; continuously withdrawing from said converter a gaseousproduct comprising said alkene, an alkenyl alkanoate, alkanoic acid, andhigh-boiling byproducts; separating said gaseous product into afixed-gas component comprising predominantly said alkene and a liquidcomponent comprising'alkanoi'c acid, an alkenyl alkanoate, andhigh-boiling by-products; recovering said alkenyl. alkanoate asdistillate from said liquid component bydistillation while recoveringalkanoic acid and said high-boiling by-products as the residue streamfrom said distillation; and utilizing said residue stream as a source ofalkanoic acid to be recycled to said catalytic converter, theimprovement which comprises:

stripping alkanoicacid from said distillation residue stream, andincorporating said stripped-out alkanoic acid into said gaseous mixturewhich is to be passed through said converter, by countercurrentlystripping said distillation residue stream with the gaseous mixturecomprising alkanoic acid and said alkene being withdrawn from saidvaporization apparatus.

'2. In a process which comprises introducing liquid acetic acid and acontinuous stream of fixed gas comprising predominantly ethylene into aheated vaporization apparatus; continuously vaporizing at least aportion of said acetic acid into said ethylene stream in said apparatus;withdrawing from said apparatus agaseous mixture com-prisingpredominantly ethylene and acetic acid; passing said gaseous mixture inadmixture with molecular oxygen through a catalytic converter containinga Group VIII noble metal catalyst; reacting ethylene, acetic acid, andoxygen in the vapor phase, in said converter in the presence of saidcatalyst of form vinyl acetate; continuously withdrawing from saidconverter a gaseous product comprising ethylene, vinyl acetate, aceticacid, and high-boiling by-products including acetoxyvinyl acetate;separating said gaseous product into a fixed-gas component comprisingpredominantly ethylene and a liquid component comprising acetic acid,vinyl acetate, and high-boiling by-products; recovering vinyl acetate asdistillate from said liquid component by water-azeotropic distillationwhile recovering acetic acid and said highboiling by-products as theresidue stream from said azeotropic distillation; and utilizing saidresidue stream as a source of acetic acid to be recycled to saidcatalytic converter, the improvement which comprises:

stripping acetic acid from said azeotropic distillation residue stream,and incorporating said stripped-out acetic acid into said gaseousmixture which is to be passed through said converter, bycountercurrently :stripping said azeotropic distillation residue streamwith the gaseous mixture comprising acetic acid and ethylene beingwithdrawn from said vaporization apparatus.

3. The improvement of claim 2 further characterized in that the strippedliquid residue resulting from countercurrently stripping said azeotropicdistillation residue with said gaseous mixture comprising ethylene andacetic acid is allowed to pass into said vaporization apparatus, fromwhich there is ultimately withdrawn a liquid vapori- 8 zation residuecomprising acetic acid and said high-boiling by-products.

4. The improvement of claim 3 further characterized by carrying out saidcountercurrent stripping by continuously introducing said azeotropicdistillation residue into the upper end of a stripping column whilecontinuously introducing said gaseous mixture comprising ethylene andacetic acid from said vaporization apparatus into the lower portion ofsaid stripping column and forwarding stripped liquid from the base ofsaid column to said vaporization apparatus.

5. The improvement of claim 4 further characterized in that arectification section is superimposed atop said stripping column, withfresh acetic acid substantially free of said high-boiling reactionby-products being introduced into the top of said rectification section.

6. The improvement of claim 5 further characterized in that the pressuremaintained in said vaporization apparatus and said column issubstantially the same as that prevailing in said catalytic converterand heat is supplied to said reboiler in an amount and at a temperaturesufiicient to vaporize all of the acetic acid contained in said strippedazeotropic distillation residue stream while maintaining in saidvaporizer enough liquid acetic acid to provide a vaporization residuecontaining at least about 50% acetic acid by weight.

7. The improved process of claim 6 wherein the vaporization apparatusand the column are operated at a pressure of about 6 to 10 atmospheres;heat is supplied to the vaporizer in an amount sufficient to control theacetic acid content of the vaporization residue between about and aboutby weight; and per unit weight of ethylene-comprising fixed gasintroduced into the vaporizer there is introduced into the combinedrectification and stripping sections of the column about 0.5 to 0.6 unitweight of acetic acid.

8. The improved process of claim 7 wherein the stripping andrectification sections of the column constitute together the equivalentof at least about 10 trays, of which at least about 5 are in thestripping section and at least about 5 in the rectification section,with about 15% to 50% of the total introduced acetic acid beingintroduced into the top of the column and the remainder being introducedat the top of the stripping section.

References Cited UNITED STATES PATENTS 3,418,215 12/1968 Nirenberg 203492,658,914 11/1953 Rigon 260-546 LORRAINE A. WEINBERGER, Primary ExaminerR. D. KELLY, Assistant Examiner US. Cl. X.R. 203 -49, 71

1. IN A PROCESS WHICH COMPRISES INTRODUCING A LIQUID LOWER ALKANOIC ACIDSELECTED FROM THE GROUP CONSISTING OF ACETIC ACID AND PROPIONIC ACID ANDA CONTINUOUS STREAM OF FIXED GAS COMPRISING PREDOMINANTLY A LOER ALKENEOF UP TO ABOUT SIX CARBON ATOMS INTO A HEATED VAPORIZATION APPARATUS;CONTINUOUSLY VAPORIZING AT LEAST A PORTION OF SAID ALKANOIC ACID INTOSAID ALKENE STREAM IN SAID APPARATUS; WITHDRAWING FROM SAID APPARATUS AGASEOUS MIXTURE COMPRISING PREDOMINANTLY SAID ALKENE AND ALKANOIC ACID;PASSING SAID GASEOUS MIXTURE IN ADMIXTURE WITH MOLECULAR OXYGEN THROUGHA CATALYTIC CONVERTER CONTAINING A GROUP VIII NOBLE METAL CATALYST;REACTING SAID ALKENE, ALKANOIC ACID, AND OXYGEN IN THE VAPOR PHASE, INSAID CONVERTER IN THE PRESENCE OF SAID CATALYST TO FORM AN ALKENYLALKANOATE; CONTINUOUSLY WITHDRAWING FROM SAID CONVERTER A GASEOUSPRODUCT COMPRISING SAID ALKENE, AN ALKENYL ALKANOATE, ALKANOIC ACID, ANDHIGH-BOILING BYPRODUCTS; SEPARATING SAID GASEOUS PRODUCT INTO AFIXED-GAS COMPONENT COMPRISING PREDOMINANTLY SAID ALKENE AND A LIQUIDCOMPONENT COMPRISING ALKANOIC ACID, AN ALKENYL ALKANOATE, ANDHIGH-BOILING BY-PRODUCTS; RECOVERING SAID ALKENYL ALKANOATE ASDISTILLATE FROM SAID LIQUID COMPONENT BY DISTILLATION WHILE RECOVERINGALKANOIC ACID AND SAID HIGH-BOILING BY-PRODUCTS AS THE RESIDUE STREAMFROM SAID DISTILLATION; AND UTILIZING SAID RESIDUE STREAM AS A SOURCE OFALKANOIC ACID TO BE RECYCLED TO SAID CATALYSTIC CONVERTER, THEIMPROVEMENT WHICH COMPRISES: STRIPPING ALKANOIC ACID FROM SAIDDISTILLATION RESIDUE STREAM, AND INCORPORATING SAID STRIPPED-OUTALKANOIC ACID INTO SAID GASEOUS MIXTURE WHICH IS TO BE PASSED THROUGHSAID CONVERTER, BY COUNTERCURRENTLY STRIPPING SAID DISTILLATION RESIDUESTREAM WITH THE GASEOUS MIXTURE COMPRISING ALKANOIC ACID AND SAID ALKENEBEING WITHDRAWN FROM SAID VAPORIZATION APPARATUS.